Jet engines operate by forcing a fluid, such as a gas, through the engine to propel the structure attached to the engine through the fluid. The process of forcing the fluid through the engine typically involves the use of fans, compressors, and turbines rotating on a shaft that extends axially through the engine. In each stage of the engine, various flow paths of gas are formed to provide either power or cooling as needed.
The turbine stages of the engine include substantially cylindrical turbine disks having a substantially cylindrical blade platform supporting the turbine blades rotating about the shaft. The rotation of the blades is at least partially caused by compressed hot gas flowing through the blades. The blades may be configured by shape or orientation to more efficiently move about the shaft in response to the flow of hot gas.
The blade disks may be positioned on opposite sides of stationary vane disks having a substantially cylindrical vane platform, which supports vanes extending radially from the vane platform. The vane platform and the blade platform each have ledges that extend axially toward each other and overlap axially to form a radial gap. The vane platforms and the blade platforms are arranged axially through the turbine portion of the engine. The vane and blade platforms together form a substantially cylindrical case allowing for a gas flow path to form on an outside surface of the vane and blade platform case and another gas flow path to form inside the vane and blade platform case.
The gas flow paths formed inside and outside the vane and blade platform case typically have a different function. In one example, the gas flow path through the vanes and blades outside of the vane and blade platform may be a hot gas flow that drives the turbine blades, while the gas flow path through the inside of the vane and blade platform case may be a cooler gas flow used to cool the disk rim areas at the gap between the vane and blade platforms. The hot gas flow temperatures typically exceed the capability of the components in the disk rim areas of the rotating disks. The cool gas flow is used to cool the disk rim areas.
The cool gas flow is a secondary flow cooling air taken after it has passed through the compressor of the engine. Higher pressure air is typically needed to purge the many cavities located around the turbine disk rims under the vane platforms. Sufficient pressure is needed so that the secondary flow air flows through the disk rim cavities and into the turbine flow path and actively purge the disk rim cavities. The active purge flow prevents the hot gas flow from entering the disk rim cavities and heating up the turbine disks and spacers beyond their allowable material-based temperature limits.
The gaps that exist between the turbine blade and vane overlaps need to be sized to avoid a rub with today's typical turbine engine configurations. A significant rub at these locations can be damaging to the turbine blade platforms and can potentially lead to platform overhang creep and/or cracking and the leading or trailing edge of the platform can actually be liberated in extreme cases. To minimize the potential for a turbine blade and vane to rub at the platform overlaps, the nominal gap needs to be sized larger than otherwise necessary due to the dimensional tolerance stack-up in the components. This larger than required gap leads to the need for additional secondary flow air to be used to positively purge the disk rim cavities and prevent flow path ingress. This additional secondary flow air is basically an efficiency reduction for the turbine and results in higher secondary flow cooling for the engine at a given operating power. If this platform overlap gap can be minimized to remove the effects of tolerances on the components the secondary flow cooling air can thus be minimized with associated benefits to engine performance.